1. Field of the Invention
The present invention relates to a technique to perform an effective heat radiation in a semiconductor device having a SOI (Silicon on Insulator) structure.
2. Description of the Background Art
Generally, a semiconductor device having a SOI structure (SOI device) has a silicon layer (SOI layer) formed on a support substrate through a silicon oxide film (SiO2). Accordingly, a semiconductor element such as, for example, a transistor and so on, formed on the SOI layer, has a structure that its periphery is covered with the silicon oxide film. A thermal conductivity of the silicon oxide film is extremely low as compared with silicon (Si) constituting the support substrate, aluminum (Al) employed for wirings and so on. Therefore, with regard to the SOI device, a heat which is generated in the transistor formed on the SOI layer is hard to let off outside, and a phenomenon that a temperature of the transistor rises and a flowing current decreases (self-heating effect) occurs. When a current flowing in the transistor drops caused by the self-heating effect, instability and malfunction of an operation of the SOI device can occur. Accordingly, a technique to raise a heat-radiating effect of the SOI device is conventionally suggested.
For example, a technique to let off the heat generated in the SOI layer to the support substrate by forming a trench isolation and a contact which are in contact with the SOI layer and the support substrate for the heat radiation is suggested (Japanese Patent Application Laid-Open Nos. 10-50999 (1998), pp. 3 to 5, FIGS. 1 and 2, 11-354807 (1999), pp. 4 to 7, FIG. 1, 2002-124564, pp. 3 and 4, FIG. 2, 2002-198493, pp. 3 and 4, FIGS. 1 to 7 and 5-267443 (1993), pp. 3 and 4, FIGS. 1 to 20, for example). Moreover, there is also a technique to remove the support substrate in a SOI substrate and join an oxide film below the SOI layer directly with an upper surface of a radiating fin (Japanese Patent Application Laid-Open No. 6-310633 (1994), pp. 4, FIG. 2, for example). A technique to raise the heat-radiating effect by forming an alloy layer with diffusing a metal on the support substrate is also suggested (Japanese Patent Application Laid-Open No. 2-303141 (1990), pp. 2 and 3, FIG. 1, for example). Moreover, there is also a technique to raise the heat-radiating effect by making the support substrate thin by a polishing and forming a metal film below (Japanese Patent Application Laid-Open No. 4-356967 (1992), pp. 3, FIG. 1 to 6, for example).
Furthermore, there is also a technique to form a metal film penetrating the oxide film from a side of the support substrate and reaching an under surface of the SOI layer and let off the heat generated in the SOI layer to the metal film (Japanese Patent Application Laid-Open No. 6-29376 (1994), pp. 4 to 6, FIGS. 1 to 7, for example). According to this method, the heat generated in the SOI layer is let off directly to the metal film without the oxide film or the support substrate, thus a high heat-radiating effect can especially be expected.
The high heat radiating effect can be expected by forming the metal film being directly in contact with the under surface of the SOI layer such as Japanese Patent Application Laid-Open No. 6-29376 described above. However, with regard to a recent SOI device with a view of a high-speed operation (high-speed SOI device), a source/drain diffusion layer in the transistor of the SOI layer has a structure reaching the under surface of the SOI layer. According to that, a junction capacitance in the source/drain diffusion layer is controlled to be low, thus the high-speed operation becomes possible. When the metal layer being in contact with the under surface of the SOI layer having such a structure is formed, a source and a drain of the transistor are shorted through the metal layer.